Sources:
http://amrita.vlab.co.in/?sub=1&brch=74&sim=202&cnt=1

A ballistic pendulum measures a bullet’s momentum, thus it is possible to calculate the velocity and kinetic energy. It is used to measure any transfer in momentum. The mass of the bullet, mass of pendulum, maximum height reached by pendulum, velocity of the bullet, velocity of pendulum are all required measurements. Physics principals such as: inelastic collisions, kinetic energy, momentum, potential energy, and the law of conservation of momentum are all involved.
BEat.jpg
Pictured above is a ballistic pendulum.
bpen.gif
The picture above explains the physics in motion of the pendulum.
Conservation_Laws15.gif
The picture above shows the parts making up the pendulum.